This invention relates to the location of pulse echo radar emitters from a moving observer by comparing measured Doppler shifts in the radar's signal. It also relates to the identification and classification of the emitters.
It has been found to be desirable to make both time and frequency measurements on a radar signal in order to locate an emitter in both angle and range. The emitter may be in a mobile system, so that its location varies from day to day, or even hour to hour. The observer may be an unmanned aerial vehicle (UAV) with only a rudimentary navigation system, or alternatively the observer may be a high performance aircraft. Particularly, it is desirable that high performance aircraft be able to update the current emitter location using parameters based on frequency and time measurements made by the UAV or other observer, such as an AWACS aircraft, at some previous time and with the emitter in a different location. Such parameters, if an intrinsic part of the location technique, must not be susceptible to either inadvertent or calculated change by the radar.
To locate emitters from such diverse platforms it is desirable to require little interface with existing systems on a platform and make no modifications to the observer's airframe. In particular, it is desirable to use existing antennas already installed on the airframe and to make minimum demands on information from the aircraft's navigation system.
The use of existing antennas imposes a significant constraint: only a single antenna should be used to make the measurements from which emitter location is derived. This is because if two antennas are required to simultaneously receive signals it is difficult to assure a common field-of-view, and hence coverage around the aircraft can be too restrictive.
U.S. Pat. No. 5,241,313 to Shaw, et al, entitled "Angle of Arrival Measurement via Time Doppler Shift" is of interest, since it describes an emitter location method requiring only one antenna. Although the patent discusses emitter angular location, it is clear that the emitter can be located in range as well, using standard bearings-only passive ranging methods. Shaw, et al, measure the Doppler change associated with radar pulse times-of-arrival (TOA). The emitter angular location is derived after the observer flies precise "dogleg" patterns, with a known speed and heading. But UAVs may not have navigation (NAV) systems that measure velocity with sufficient accuracy to be usable in this approach. UAVs typically measure speed with relatively more precision, but heading may be measured too crudely to provide a basis for extracting signal angle-of-arrival with the accuracy needed for bearings only passive ranging.
The approach of Shaw et al also requires multiple dwells, and signal coherence across these dwells, to obtain a range solution. But when a fire control radar is engaging an attacking aircraft, the radar transmitter may be "blinked", or turned on and off as a defense against anti-radiation missiles. When the radar is blinked, signal coherence can be lost.